An example of such ophthalmic examination systems is disclosed in Patent Literature 1. The arrangement disclosed in this Patent Literature 1, the one according to the third embodiment shown in its FIG. 9, in particular, is very simple and includes a light source for presenting the visual target and a light source for presenting the background to the target. The target light source is arranged to blink, and the light beam from this light source is collimated by a collimator lens. After that, only an S (Senkrecht) polarization component is extracted from the collimated light beam by a polarizing plate, before it impinges on the incident surface of a liquid crystal shutter. The liquid crystal shutter includes shutter pattern including a transmissive region, e.g. a circular region, having an area sufficiently smaller than the effective area of the incident surface, and a remaining, blocking region. Only the light (S-polarized light) beam incident on the transmissive region of the incident surface provided with the shutter pattern passes through the liquid crystal shutter and emerges out from the emerging surface. The emerging light beam is arranged to be incident on a polarizing prism acting as combining means.
The background presenting light source has an arrangement similar to that of the target presenting light source and emits a light beam of a constant intensity same as the target presenting light source when it is on. The light beam emitted from this background presenting light source is collimated by another collimator lens. After that, only an S (Senkrecht) polarization component is extracted by another polarizing plate, before it impinges on the incident surface of another liquid crystal shutter. The liquid crystal shutter has a shutter pattern reverse to the one described above. Only the light beam (P-polarized light beam) incident on the transmissive region, corresponding to the above-described blocking region, of the incident surface provided with the shutter pattern, passes through the liquid crystal shutter and emerges out from the emerging surface. The emerging light beam, too, is incident on the above-described polarizing prism.
The polarizing prism has the boundary surface receiving the light beam emerging from each of the liquid crystal shutters, and emerging light beams are incident onto the boundary surface at angles, which are conjugate relative to each other. One of the incident light beams, i.e. the S-polarized light, is reflected at the boundary surface, whereas the P-polarized light beam for the background passes through the boundary surface. Thus, the S-polarized light beam reflected by the boundary surface and the P-polarized light beam passing through the boundary surface are combined. The resultant light beam is projected through a magnifying optical system onto the eye to be examined.
As a result, the blinking visual target and the background having the same brightness as the target when it is on are presented to the eye to be examined. In other words, the target is presented with brightness equal to or less than that of the background. Accordingly, when the target starts to blink, the amount of light incident on the eye to be examined does not increase, and, accordingly, a suppressed stimulus is given to the eye under examination. In this manner, erroneous recognition by the person receiving the examination caused by stimulus to the eye is reduced. (In other words, possibility of erroneously recognizing the target as if it were blinking at the instant the target just starts blinking.) Thus, the reliability of examination increases.
This feature is significantly advantageous in subjective examination, e.g. a visual field examination, for example. In subjective examinations including a visual field examination, a person receiving the examination operates a response button, for example, to notify an examiner that he or she has recognized the blinking of the target. Like this, the subjective examination in which the examination is based on such subjective indication by the person receiving the examination, and, therefore, the described prior technique in which the possibility of erroneous recognition can be reduced, is significantly effective for increasing the reliability (or precision) of the examination.
Patent Literature 1: JP 2006-340755A (Page 10, and FIG. 9)